In human computer interface, as well as robotic control and other fields, it is often desirable to make sensors that measure the distribution of forces applied to a surface of an object by physical contact with other objects. In robotics applications, these measurements may allow a manipulator with a sensing surface to be adjusted so that the objects are handled properly. When the objects are human fingers, a force sensor can be used as an interface, allowing a person to control computer software such as music applications.
In a musical application, trained musicians are able to control sounds by using gestures that are extremely rapid, accurately placed, and may range in pressure from very light to very heavy. Accordingly, a sensor used to control sound in musical applications should sense a wide range of forces, have a high temporal resolution, and have high positional accuracy. Such a sensor should ideally also be portable, rugged and inexpensive to manufacture.
A mechanical approach to a musical sensor is taught by U.S. Pat. No. 6,703,552 B2 issued to Haken. Haken's Continuous Music Keyboard comprises a plurality of rods, each of which has a magnet on each end. Pressing on a control surface displaces the rods and allows surface force displacement to be measured. Although this device has high temporal resolution and positional accuracy, its sensing method allows only one force at any given horizontal position to be recognized. In addition, this mechanical approach to sensing results in a device that is relatively heavy and expensive to produce.
It is well known that the capacitance between a pair of parallel plate conductors varies inversely as the distance between the conductors. This principle has been used to construct a variety of capacitive sensors. Capacitive force sensors can be made using conductive layers separated by an elastic dielectric. When forces are applied to these sensors, the distance between the conductive layers changes. This distance can be determined by measuring the change in capacitance. Using multiple capacitive elements, sensors can be made that determine the distribution of forces on a surface.
For example, U.S. Pat. No. 4,526,043 issued to Boie et al. discloses a force sensor including an elastic dielectric, a first plurality of conductive elements on one side of the dielectric and a second plurality of conductive elements on the opposite side of the dielectric. At each point where the conductive elements overlap, the capacitance between the two elements can be measured to determine the distance between them, and thereby the force applied to the sensor's surface. When a measurement is to be taken of the spacing between the elements at a given point, a voltage signal is routed to a first element using an analog multiplexer, and a signal from an overlapping second element is routed to a phase-sensitive detector circuit using a second analog multiplexer. These multiplexers make connections between a given circuit element and one from a group of other elements, one such connection at a time. This widely used technique is known as time-division multiplexing.
U.S. Pat. No. 4,827,763 issued to Bourland et al. discloses a pressure distribution measuring system including a pad of insulating material with a central array of linear electrodes, aligned perpendicular to two outer linear electrode arrays. This system also incorporates a time-division multiplexing circuit to attach a measuring apparatus sequentially to each of multiple sensing elements.
U.S. Patent Application No. US 2008/0127739 A1 of DeAngelis et al. describes a flexible capacitive sensor with a resilient dielectric. One embodiment of this invention teaches the use of multiple capacitive elements to determine the position of a force, again only by the use of time-division multiplexing.
Further patents in the field of capacitive sensing, including U.S. Pat. No. 4,644,801 issued to Kustanovich, U.S. Pat. No. 6,862,942 B2 assigned to Kawahata, U.S. Pat. No. 6,826,968 assigned to Manaresi et al., and U.S. Pat. No. 4,389,512 assigned to Speck, all teach the use of a time-division multiplexing circuit as the only method of switching between sensing elements.
Though time-division multiplexing is the most widely-used technique to connect multi-element sensors to signal generation and signal processing elements, it suffers from a number of disadvantages:
(a) Measurements taken by different elements are not simultaneous. Because only one element of a multiplexed group of elements can be measured at any given time, errors can occur in the relative timing of force measurements from different elements. In particular, two forces simultaneously applied at different locations may be mischaracterized as having occurred at different times. In a musical application, these temporal errors may be perceptible and thereby have a negative impact on playability. In a robotics application, they may limit the temporal precision of control.(b) Measurements are not continuous. Transient forces of short duration may go entirely undetected if applied to a particular element while other elements are being measured.
These disadvantages of time-division multiplexing can be minimized by increasing the switching rate of the multiplexing. However, increasing the switching rate also necessarily decreases the time intervals during which each element can be measured. As the rate increases, more sensitive and thereby more expensive detection circuitry is required.
U.S. Pat. No. 6,498,590 issued to Dietz et al. teaches the use of a code-division multiplexing scheme to distinguish between antennas in a multi-user capacitive touch sensing system. This approach is one solution to the problem of making detectors that work at higher switching rates. In this system, a plurality of carrier elements is driven by a digital pseudo-random noise sequence generated using a polynomial function passed through a shift register to provide time delays. This patent also briefly mentions the possibility of frequency-division multiplexing, an approach in which multiple sensing elements are uniquely identified with different carrier frequencies, but dismisses this approach as being relatively expensive.